Alkaline phosphatase represents a developmentally regulated enzyme whose activity appears at a particular time and location in sea urchin embryogenesis. The enzyme activity is induced in post-gastrulation embryos and is localized primarily to the developing gut. Thus alkaline phosphatase represents a marker for the formation of a complex morphological structure in early development. Experiments are proposed to determine where and when alkaline phosphatase and its mRNA are synthesized during embryo development. A method for isolation of a cDNA clone complementary to alkaline phosphatase mRNA is described, relying on in vitro translation of alkaline phosphatase mRNA and detection of the alkaline phosphatase peptide chain using immune precipitation. The cloned probe will be used to monitor the accumulation of alkaline phosphatase mRNA sequences in the nucleus, cytoplasm and polysomes of the developing embryo. The cDNA clone will also be used to screen a charon 4 lambda-sea urchin DNA library for the genomic DNA containing the alkaline phosphatase gene. The location of repetitive and single copy DNA sequences on this lambda-recombinant will then be determined.